Chicago Loop is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Start your engines! : Add . Pay X speed: Add X mana in any combination of colors. , : Create your choice of a 3/1 green Dinosaur Skeleton creature token with trample, a 2/2 orange Bear token with haste, or a 1/1 white Bird token with flying.